As a piezoelectric material, PZT (a PBZrO3—PbTiO3 solid solution), which is a ceramic material, has been used in many cases. However, PZT contains lead, polymeric piezoelectric materials with low environmental load and a high degree of flexibility have been increasingly employed.
Currently known polymeric piezoelectric materials are roughly classified mainly into the following two types, i.e., poled polymers typified by nylon 11, polyvinyl fluoride, polyvinyl chloride, and polyurea, polyvinylidene fluoride (β type) (PVDF), and ferroelectric polymers typified by a polyvinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) (75/25).
In recent years, use of polymers having optical activity, such as polypeptide or polylactic acid, is attracting attention, in addition to the polymeric piezoelectric materials described above. Polylactic acid-based polymers are known to demonstrate piezoelectricity by carrying out a mechanical stretching alone.
Among the polymers having optical activity, piezoelectricity of polymer crystals, such as polylactic acid, is supposed to originate from permanent dipoles of a C═O bond being present in a screw axis direction. In particular, polylactic acid, which has a small volume fraction of a side chain to a main chain and a high ratio of permanent dipoles per volume, is an ideal polymer among polymers having helical chirality.
It is known that polylactic acid, which demonstrates piezoelectricity by a stretching process alone, does not require a poling process, and that the piezoelectric modulus is less likely to decrease over the years.
As a device using a polylactic acid-based polymer as a polymeric piezoelectric material, a touch panel provided with a piezoelectric sheet of polylactic acid is proposed, for example (see, for example, WO2010/143528, WO2011/125408, WO2012/049969, and WO2011/138903).